The present invention relates to a code-division multiple-access receiver.
Code-division multiple-access (CDMA) communication enables multiple transmitting and receiving stations to share the same channel: for example, the same frequency band in the radio spectrum. In one type of CDMA system, each transmitting station has a different spreading code with which it spreads a baseband signal containing symbols to be transmitted. A receiver can recover the symbols transmitted by a particular station by using a corresponding despreading code. The despreading code singles out the signal of the desired station and reduces the signals of other stations to thermal noise.
Advantages of CDMA include security, resistance to multipath fading, and efficient utilization of bandwidth. The number of stations that can share the same channel is limited, however, by co-channel interference, which arises from a lack of synchronization between different transmitters, or a lack of orthogonality between different spreading codes or despreading codes. In a cellular telephone system, for example, such interference limits the number of mobile stations that can access the same base station at once. As the number of stations increases, the bit error rate also increases, until communication quality is degraded to all unacceptable degree.
Realization of the full potential of CDMA communications requires receivers that can cancel co-channel interference effectively, to achieve a low bit error rate even when many stations are active simultaneously. Among the proposed systems is one that operates on the signals from different stations in parallel and iteratively, canceling estimated interference at each iteration. The parallel architecture of this system, however, requires a large amount of memory, and simulation shows it to be less effective than expected. One reason is that in each iteration stage, no interference cancellation takes place until processing of all stations' signals is completed.